Degeneration and regeneration of nerve terminals in the rat pituitary pars intermedia after 6-hydroxydopamine treatment

The mammalian pituitary intermediate lobe: an update on innervation and regulation

The pituitary intermediate lobe (IL) in mammals is an area of uniform endocrine cells which synthesize and release specific peptide products of the proopiomelanocortin gene. The lobe receives direct synaptic connections onto the endocrine cells from hypothalamic dopaminergic neurons. This review updates information on the dopaminergic as well as the gamma-aminobutyric acid inhibitory neuroregulation for the IL. It also provides a discussion of stimulatory molecules which are likely to affect peptide release, particularly the neurotransmitter serotonin, which may be present via uptake into the dopaminergic nerve terminals. Other stimulatory molecules discussed which are likely to significantly affect peptide secretion are norepinephrine, corticotropin-releasing factor, and several opiate peptides. A new direction of study involves the potential interaction of neurotrophic factors, which are present in all areas of the pituitary, and may be suggested to have a supportive role for the neural elements of the IL. The endocrine cells of the IL and their direct hypothalamic innervation are considered to be an easily accessed peripheral model for study of both neural-endocrine and neurotrophic-target cell interactions.

Glial cell line-derived neurotrophic factor in the rat pituitary gland

The presence of glial cell line-derived neurotrophic factor (GDNF) is described within specific regions of the adult rat pituitary gland. Immune staining methods revealed a small number of GDNF-immunopositive cells in the anterior lobe, and in areas of the neural lobe, while no immunoreactive endocrine cells were observed in the intermediate lobe. In the neural lobe, immunofluorescence methods were also used to demonstrate that GDNF and glial fibrillary acidic protein (GFAP) are co-localized in the glial cells (pituicytes) of the neural lobe. GDNF was not co-localized with neurofilament (NF) in nerve fibers of the neural lobe, suggesting that it is not present in axonal fibers. Measurements of GDNF content in separated anterior and neurointermediate lobes were also performed, using an enzyme-linked immunoassay (ELISA). Values for GDNF were slightly higher in the neurointermediate lobe than those obtained for the anterior lobe. The presence of GDNF in areas of the pituitary is discussed in the context of its possible function to support and maintain hypothalamic innervation, as well as a potential autocrine factor within endocrine cells.

Regulation of pituitary beta-endorphin secretion in aging rats: in vitro responsiveness to dopamine

The intermediate lobe of the mammalian pituitary is highly responsive to dopamine inhibition of beta-endorphin secretion. In this study, the ability of aged (12 months) intermediate lobes to respond to dopamine was compared to that of young (6 weeks) tissue, using a short-term in vitro incubation of isolated rat neurointermediate lobes, with measurement of peptide release by radioimmunoassay. Tissue from the aged rats released greater amounts of beta-endorphin peptide than amounts measured from young tissue at all time periods studied. The aged lobes were also found to be significantly more sensitive to dopamine than young glands, as measured by percent change of each group compared to respective baseline release. In comparison, incubation of tissue from young animals in which the intermediate lobe had been acutely denervated by treating rats with injections of the catecholamine neurotoxin, 6-hydroxydopamine, did not differ in responsiveness to dopamine as compared to tissue from control rats. The observations suggest that aging intermediate lobe, while being hypersecretory, is supersensitive to dopamine, perhaps as the result of gradually reduced innervation.